Pregnancy enables antibody protection against intracellular infection

Adaptive immune components are thought to exert non-overlapping roles in antimicrobial host defence, with antibodies targeting pathogens in the extracellular environment and T cells eliminating infection inside cells 1 , 2 . Reliance on antibodies for vertically transferred immunity from mothers to babies may explain neonatal susceptibility to intracellular infections 3 , 4 . Here we show that pregnancy-induced post-translational antibody modification enables protection against the prototypical intracellular pathogen Listeria monocytogenes . Infection susceptibility was reversed in neonatal mice born to preconceptually primed mothers possessing L. monocytogenes -specific IgG or after passive transfer of antibodies from primed pregnant, but not virgin, mice. Although maternal B cells were essential for producing IgGs that mediate vertically transferred protection, they were dispensable for antibody acquisition of protective function, which instead required sialic acid acetyl esterase 5 to deacetylate terminal sialic acid residues on IgG variable-region N -linked glycans. Deacetylated L. monocytogenes- specific IgG protected neonates through the sialic acid receptor CD22 6 , 7 , which suppressed IL-10 production by B cells leading to antibody-mediated protection. Consideration of the maternal–fetal dyad as a joined immunological unit reveals protective roles for antibodies against intracellular infection and fine-tuned adaptations to enhance host defence during pregnancy and early life.